ping in_cksum() calculates bad checksum on big endian machines, FIX

[Doug Kingston]

Index:	etc/ping.c 4.3BSD FIX

Description:
	The internet checksum routine in ping.c will not calculate
	the correct checksum when used on big endian machines like
	Suns and Goulds and the packet length is odd.  The problem
	is that bytes are ordered differently in shorts on these machines.
	The special case code for the trailing odd byte is broken.

Repeat-By:
	Compile ping.c on a big endian machine and say "ping host 111"
	and watch no pings return.  The remote end will start tallying
	checksum errors in the ICMP code (visible with netstat -s).

Fix:
	The bug has to do with how the odd byte is added into the sum.
	I have reproduced the entire routine below with the fixed "if"
	statement.

	The old version looked like this:
		if( nleft == 1 ) {
			sum += *(u_char *)w ;

	The new version (correct) version is given below.

#include <sys/types.h>

/*
 *			I N _ C K S U M
 *
 * Checksum routine for Internet Protocol family headers (C Version)
 *
 */
in_cksum(addr, len)
u_short *addr;
int len;
{
	register int nleft = len;
	register u_short *w = addr;
	register u_short answer = 0;
	register int sum = 0;

	/*
	 *  Our algorithm is simple, using a 32 bit accumulator (sum),
	 *  we add sequential 16 bit words to it, and at the end, fold
	 *  back all the carry bits from the top 16 bits into the lower
	 *  16 bits.
	 */
	while( nleft > 1 )  {
		sum += *w++;
		nleft -= 2;
	}

	/* mop up an odd byte, if necessary */
	if( nleft == 1 ) {
		*(u_char *)(&answer) = *(u_char *)w ;
		sum += answer;
	}

	/*
	 * add back carry outs from top 16 bits to low 16 bits
	 */
	sum = (sum >> 16) + (sum & 0xffff);	/* add hi 16 to low 16 */
	sum += (sum >> 16);			/* add carry */
	answer = ~sum;				/* truncate to 16 bits */
	return (answer);
}

============ End of Fix ==============